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Three moons of Saturn in Cassini's latest image

Three of Saturn's moons captured by the spacecraft Cassini – Tethys (centre), Hyperion (upper left) and Prometheus, along the edge of Saturn's outermost ring, or the F-Ring.

NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE

The Cassini spacecraft has captured a rare photo of three of Saturn's moons – the round Tethys (centre), the chaotically spinning Hyperion (upper-left) and the tiny elongated Prometheus (lower-left) defining the planet's F ring, its outermost ring.

None of the moons is large. Tethys is 1,062 km across, Hyperion 270 km and Prometheus just 86 km.

Cassini has uncovered much new information about the three moons. They are each very different from each other.

Tethys shows a variety of terrains. To the north of its trailing is older, rougher terrain, while to the south is new material dubbed "smooth plains" by scientists. NASA has more images and information here.

Hyperion is irregularly shaped and tumbles through space. It is covered with closely packed and deeply etched pits, which scientists believe have been caused by the warming of the Sun of water ice lying beneath a darkened layer of surface material.

Cassini scientists now believe that Hyperion’s unusual appearance can be attributed to the fact that it has an unusually low density for such a large object, giving it weak surface gravity and high porosity. There is more information here.

Prometheus is a so-called "shepherd" moon, the gravity of which helps maintain the sharply defined edge of the F-Ring. NASA has more images and information on it here.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission.